Regular 3.5 inch hard drives, hundreds of them, with some software raid to deal with dead drives. All connected by USB because spinning rust can barely fill

Regular 3.5 inch hard drives, hundreds of them, with some software raid to deal with dead drives. All connected by USB because spinning rust can barely fill USB3 bandwidth anyway. This setup is cheap to begin with (just $50 gets you your first few TB's), and scales well (127 USB devices per host)